A Tale of Fish and Stones

For Vincent P. Palace, head of the Environmental Center for Research on Pesticides at Freshwater Institute in Winnipeg, Manitoba, Canada, success is not a fish tale. Instead, it is a fish otolith — a stonelike structure in the ear composed of calcium carbonate and a protein matrix. Examining otoliths using laser ablation inductively coupled plasma mass spectrometry is helping Palace and his colleagues there and at the University of Manitoba in Winnipeg and at Alberta Sustainable Resource Development in Edson, Canada, settle a pollution question.

Researchers used otoliths from rainbow trout to analyze absorption of zinc and selenium, thereby indicating whether the fish were raised in selenium-polluted waters. The above image shows the path created by laser ablation prior to inductively coupled plasma mass spectrometry analysis of the chemicals. The otolith measures ∼1.9 mm across from ventral to dorsal tip. Images reprinted with permission of the American Chemical Society.There are streams in Alberta that have high concentrations of selenium, a byproduct of coal mining. The chemical is a necessary dietary supplement, but too much during embryo development causes deformities.

However, the waters downstream of the province’s mines were not empty of fish, leading to one of two conclusions. The first was that the selenium was not having an excessively detrimental effect. Palace said that he and his colleagues thought that another scenario was more likely. “We had always felt that there was a chance that adult fish in the system were recent immigrants and may not have been derived from eggs that were laid in the system.”

The scientists examined fish from waters that were downstream of selenium-producing mines (Luscar Creek, Alberta, Manitoba, Canada) and from a reference site (Deerlick Creek). The fish were captured in 2001, and the otolith sections indicated their age.To distinguish between the two possibilities, the researchers looked into when the fish became exposed to selenium — and that is where otoliths played a role. As fish grow, their otoliths accrete layers at a rate that varies with the season. Because the layers are deposited at a given time and are not affected by subsequent conditions, otoliths preserve traces of any environmental exposure.

However, in three to five years, otoliths grow only 1 to 2 mm, so the researchers turned to laser ablation inductively coupled plasma mass spectrometry. “This method allows us to resolve chemical differences at low concentrations — parts per million levels — and also within very small areas of the otolith growth ring structures,” Palace said.

The scientists used a high-resolution inductively coupled plasma mass spectrometer made by Thermo Electron, part of Thermo Fisher Scientific of Waltham, Mass., along with an Nd:YAG laser from New Wave Research Inc. of Fremont, Calif. They collimated the beam down to a 30-μm spot size, ablated the otolith material as the beam moved and analyzed the ablated material that resulted. Because of the spot size and the ability to track other elements, such as zinc, they could rapidly evaluate selenium exposure within the span of a single year’s worth of growth.

They report in the May 15 issue of Environmental Science & Technology that the rainbow trout in the waters downstream of the mines were late arrivals. Moreover, they found that the fish in a nearby stream — one not exposed to mining effluents — had no detectable selenium. Thus, fish were migrating into the problem stream, but none were emigrating — not surprising given the high deformity rates of rainbow trout eggs that were exposed to that much selenium.

Research continues on how selenium is affecting fish productivity within the streams, something that is not yet clear because of another wrinkle. “There are brook trout in the system, and they seem to be less affected. We have collected otoliths from this species as well and are in the process of analyzing them,” Palace said.